Processes for electroplating metallic coatings onto objects are known in the art. Electroplating is typically employed to alter the existing surface properties of an object. For instance, electroplated coatings are commonly applied to improve resistance to corrosion or abrasion, or to impart desirable electrical or magnetic properties to an object.
An electroplating process involves the following steps. The object to be electroplated is attached to a plating apparatus which serves as a cathode terminal. The plating apparatus is then immersed into an electrolytic solution. An electrode is immersed into the same solution and is electrically connected as an anode. A direct current is then applied to the electrolytic solution, thereby dissociating positively charged metal ions at the anode. The ions then migrate to the cathode, where they plate the object attached to the cathode.
Electroplating processes are commonly employed to deposit metals during the fabrication of high component density electrical devices. For example, electroplating processes are used while fabricating semiconductor dies and thin-film interconnections.
By way of example, FIG. 1 depicts a wet contact electroplating apparatus 20 which is used during thin-film interconnection fabrication. The wet contact electroplating apparatus 20 includes a number of arms 22A, 22B, 22C, and 22D for holding a wafer 24. The wafer 24 may include an aluminum base upon which conducting patterns are formed between insulating layers. For example, a layer of metal may be deposited on the base. The metal may then be etched into a predetermined conducting pattern. Then, a layer of polyimide may be formed over the conducting pattern. Thereafter, a chromium/copper layer may be sputtered onto the polyimide to prepare the surface for the electrodeposition of another layer of metal. After the electrodeposition of the metal and its etching into another conducting pattern, the process is repeated.
Regardless of the type of object which is subject to electrodeposition, a number of shortcomings are associated with a wet contact apparatus of the type disclosed in FIG. 1. A number of steps are required to prepare the apparatus 20 for electrodeposition. First, the metallic apparatus 20 must be sheathed in a non-conducting substance, such as plastic. Afterwards, the non-conducting substance must be scrapped at its arms 22 to expose the metal for electrical connection. Subsequently, the wafer 24 is positioned within the arms 22 and the apparatus 20 is immersed into an electrolytic solution.
During the electrodeposition process the metal on the arms 22 of the apparatus 20 is exposed to the electrolytic solution. Thus, a plated mass is formed as the electroplated substance accumulates on the arms 22. This plated mass disrupts the electrical characteristics of the cathode. Consequently, the plated mass may cause a non-uniform deposit on the wafer 24.
After the plating process, the plated mass on the apparatus 20 must be removed. This typically involves using a toxic stripping solution. Subsequently, specially tailored environmental steps must then be taken to dispose of the stripping solution.
As previously mentioned, a non-uniform deposit on an electroplated object may be caused by the plated mass formed on the arms 22 of the wet apparatus 20. Non-uniform deposits may also result from the fact that a wet contact apparatus makes electrical contact at the edge of the wafer, as opposed to the surface of the wafer. Non-uniform deposits may also result from one object to the next if the objects to be electroplated are positioned at different locations in the apparatus 20. This problem is especially acute in the context of thin-film depositions, where extremely high accuracy of plated metal is required.
Another problem associated with the wet contact apparatus of the prior art is the formation of "stringers". Stringers 26 are depicted in FIG. 1. Stringers are splinter-like objects that commonly form on the edge of a wafer during thin-film fabrication. Stringers may be created in a number of ways. Stringers occasionally form because the geometry of the sputtering equipment prevents a direct application of sputtered copper to the edge of a wafer. Consequently, the adhesion of the sputtered copper on the wafer edge is poor, commonly causing the copper to peel. This problem is exacerbated as the plating step builds up any peeling segment. Stringers may also be formed when the arms 22 rub against the edge of the wafer 24.
Regardless of the cause of their formation, stringers create a number of problems. If the stringers bridge onto the face of the wafer they may create a short circuit on the device. Detached stringers may accumulate and cause equipment shorting and other processing problems which are known in the art.